Electroless metallization (or electroless plating) is a method for obtaining a thin metallic film on materials such as metals, ceramics or plastics by a series of process steps, including the use of a conditioner, microetch, catalyst (activation) and electroless plating bath. The mostly popular catalysts for electroless plating during the past 30 years are Pd/Sn colloids in which palladium colloidal particles are stabilized by tin chloride. Prior to electroless plating, the surface charge of a dielectric substrate is typically negative. Due to electrostatic repulsion, the negative charge on the substrate will typically inhibit the adsorption of catalyst which is a negative charged colloid. The initiation of electroless metallization can be difficult if there is insufficient catalyst adsorption. In order to improve catalyst adsorption, a traditional conditioner step is often used. Typical constituents of a traditional conditioner include cationic surfactants, which provide positively-charged groups to change the substrate from negatively to partially positively charged, so that the adsorption of the catalyst can be improved. In other words, the interaction between the substrate and the catalyst is enhanced by a traditional conditioner via electrostatic attraction.
The reliability of circuits relies heavily on the adhesion between the deposited metal layer and the dielectric substrate. Poor adhesion may lead to unacceptable failures, such as “peel-off” or blistering. The electrostatic attraction from the traditional conditioner may not be strong enough to build up sufficient adhesion between the deposited metal layer and the dielectric substrate. Roughening the surface of dielectric substrates, for example by using a de-smear process, is often performed before electroless plating to increase the metal adhesion. Sufficient adhesion between the deposited metal layer and the dielectric substrates is then provided by the roughness of the dielectric substrate. Good adhesion is more difficult to obtain on a smooth surface using such process flows.
As L/S for next generation chip carriers get smaller, there is a need for smoother surfaces to etch fine lines. As the width and space of signal traces continue to shrink, and the frequency of the signals continues to increase, the surface of the insulating materials need to be smoother. Achieving high adhesion of a metallized layer on low profile insulating materials becomes increasingly difficult. Traditional etching-type processes to improve adhesion are no longer feasible, thus an alternative method is needed to improve the adhesion of fine patterned circuits on insulating materials. There is a need for improvements on traditional roughening processes and development of novel approaches to attain high adhesion of metal to polymer on a smooth surface for finer line chip carriers. In order to meet the requirements of the electronics industry for fine line circuits, a conditioner needs to be capable of providing high adhesion to a smooth (low profile) surface, as well as modifying the surface charge of dielectric substrates to improve the adsorption of a negatively charged catalyst.
Marine mussel organisms are known for their ability to bind strongly to such varied surfaces as rocks, pilings, and ship hulls. The adhesive characteristics of mussel adhesive proteins are believed to be due to the presence of 3,4-dihydroxyphenylalamine (DOPA). P. B. Messersmith et al. SCIENCE, 2007, Vol 318, page 426-430 discloses the use of dopamine to mimic the ability of mussel's adhesive protein to adhere to a surface of materials. Via a self-polymerization process, a thin, surface-adherent polydopamine film can be formed onto a wide range of inorganic and organic substrates through simple dip-coating of these substrates in an aqueous solution of dopamine at room temperature for 24 hours. Electroless silver or copper deposition was performed on the polydopamine coated surfaces to form a layer of silver or copper. CN10182678A discloses a polydopamine-assisted electroless silver process, in which dip coating of dopamine for 0 to 48 hours at room temperature was used. US2010/0021748A discloses a process comprising the steps of treating a substrate with a catecholamine solution including dopamine, to form a polycatecholamine layer followed by electroless plating.
Reviewing the prior art for dopamine-aided electroless plating, it can be found that the dopamine self-polymerization process typically requires room temperature treatment for about 24 hours. Coupled with the relatively high cost of dopamine, the long dwell time of dopamine treatment is a barrier to commercial application. In addition, the pH range that is suitable for the self-polymerization of dopamine is quite narrow (6.5-9.5). Outside this pH range, the self-polymerization rate of dopamine becomes extremely low.